Retarder control systems



March 15, 1966 R. E. PORTER ETAI.

RETARDER CONTROL SYSTEMS '7 Sheets-Sheet 2 Filed Jan. 13, 1965 .wwfozOp.

INVENTORS ARTHUR R. CRAWFORD 1\CHP\RD E. PORTER M,

March 15, 1966 R, E PORTER ETAL 3,240,930

RETARDER CONTROL SYSTEMS '7 Sheets-Sheet 4 Filed Jan. 13, 1965 INVENTORSARTHUR. R. CRAWFORD \C|\/-\RD E. PORTER 5f PM ZONE v m@ l l l I I l l lNUL w zo.:

March 15, 1966 R E, PORTER ETAL 3,240,930

RETARDER CONTROL SYSTEMS '7 Sheets-Sheet 6 F'iled Jan. l5, 1965 March15, 1966 R E, PQRTER ETAL 3,240,930

RETARDER CONTROL SYSTEMS 7 Sheets-Sheet 7 Filed Jan. 13, 1965 IoJmDOw OP.M M E. mm WT ,f7 AW H MP R5 0 www AH mwhllllh 55E \0tm n.355 \\mtm n nvlm United States Patent O 3,240,930 RETARDER CONTROL SYSTEMS Richard E.Porter and Arthur R. Crawford, Columbus, Ohio, assignors to AmericanBrake Shoe Company, New York, NY., a corporation of Delaware Filed Jan.13, 1965, Ser. No. 427,537 19 Claims. (Cl. 246-182) This inventionrelates to a new and improved speedsensitive control system suitable foruse in the control of a railway car retarder or like apparatus. Thisapplication is a continuation-in-part o-f application Serial No. 228,-189, led October 3, 1962, now abandoned.

In the operation of a railroad classification yard, railroad cars areordinarily released from the top of an incline or hump to roll down theincline into the branching tracks of the classification yard.Frequently, it is necessary to provide some means for braking the carsat one or more points along the gang or feeder track. Braking may alsobe required on other tracks in the classification yard. The track brakesor car retarders used usually include one or more pairs of brake shoesof elongated rail-like construction that engage the sides of the wheelsof the cars.

In braking installations of this kind, it is frequently desirable tovary the braking force applied by the retarder to each car in accordancewith the speed of the car as it enters the retarder. Thus, if a car isrolling at a speed less than Ia predetermined safe speed for movementbeyond the retarder, it may be permitted to pass on to the remainder ofthe yard without braking; that is, the car retarder should permit thecar to pass without braking engagement. A car rolling at a speed abovethe safe classification speed, however, must be subjected to asubstantial braking pressure, the brake being relieved when the car hasbeen slowed to the desired exit speed.

Deceleration of the car in the retarder depends on a variety of factors,including the weight of the car, the condition of its bearings, moistureon the braking rails of the retarder, and the like. Thus, one car may bebraked to a safe speed after it has traversed only onehalf of theretarder. Another and heavier car entering the same retarder at the samespeed may require braking for virtually the entire length of theretarder in order to achieve a safe exit speed. For this reason,accurate and reliable measurement of the railroad car speed is highlydesirable in controlling a railroad car retarder, and it is advantageousto provide for continuous monitoring of the car speed throughout thelength of the retarder.

One system that has been employed in the past for the determination ofcar speeds, in a retarder installation, er1- tails the segmentation ofthe traflic rail along which the car rolls into a plurality ofrelatively short sections that are insulated electrically from eachother. This makes it possible to measure the speed of the car,periodically, in accordance with signals derived from engagement of therails by the car wheels, speed determination being based upon thetransit time required by the car in passing over individual sections ofthe segmented rail. Other similar arrangements have utilized a pluralityof treadles mounted along the rail and engageable by the car wheels, aplurality of photocells or sensing switches, or like means for checkingthe position and speed of the car as it enters or as it passes through acar retarder installation.

It is a primary object of the present invention to provide a new andimproved speed-sensitive control system, suitable for control of arailway car retarder or the like, that provides for reliable andaccurate speed determination on a continuous basis.

A particular object of the invention is to provide a new and improvedspeed-sensitive system for controlling a railp ice way car retarder orthe like that does not require physical segmentation of the traffic railor the provision of multiple auxiliary devices along the rail, yet whichaffords a substantially continuous spe-ed determination sensitive tosmall changes in car speed over relatively short distances.

A specic object of the invention is to provide a new and improvedcontinuous operating speed-sensitive control system in which theessential data for speed determination are obtained directly from thetraic rail of a railway, in response to vibration of the rail by anincoming Vehicle.

An additional object of the invention is to afford a new and improvedcontrol system for a railway car retarder, and particularly aspeed-determining system for continuously monitoring the speed of arailway car, which requires no moving parts at the rail itself andentails no electrical connection to the rail, yet uses the rail as theinitial input member to the system.

An additional object of the invention is to provide a new and improvedspeed-sensitive control system for a railway car retarder that does notsense or Contact any part of the car being retarded and hence isindependent of variations in car construction.

A corollary object of the invention is to provide reliable and accuratespeed control of a railway vehicle retarder, based on control apparatusthat is rugged and substantially maintenance-free, yet relativelyinexpensive in construction. v

A further object of the invention is to provide a new and improvedspeed-sensitive control system for a railroad car retarder, in which thespeed of an incoming car or cut of cars is determined by sensingvibration of a traffic rail, and to protect that system againsterroneous operation due to vibration of the traffic rail caused bysources other than the incoming cars. Typical extraneous sources oflvibration of the traffic rail include the actuation of switches on therails near the retarder, passage of cars or locomotives along adjacenttracks, and the like.

Still anotherrobject of the invention is to protect a rail-vibrationactuated speed sensing control system for a railroad car retarderagainst erroneous operation due to the presence of at portions on thewheels of cars passing through the retarder.

The present invention therefore provides a speed-sensitive controlsystem for a railway car retard-er or the like comprising: a traic railhaving a series of surface discontinuities at equally spaced intervalsalong a wheelengaging surface thereof; vibration-sensitive transducermeans, mounted on said rail, for developing an electrical signal havinga fundamental frequency determined by the velocity of a car wheelengaging said surface discontinuif ties; and electrical control means,responsive to said sig# nal, for actuating said retarder between brakingand released conditions in response to variations of said fundamentalfrequency above and below a frequency representative of a given releasevelocity.

In the preferred construction described hereinafter, the surfacediscontinuities in the traic rail comprise shallow linear grooves formedtransversely of the rail, but they could also constitue notches, pits,or grooves in the central bearing surface or in the outside corner ofthe rail, or ridges on the rail, if desired. The frequencysensitivecontrol means employed in theV system may t-ake several different forms,as described in detail hereinafter. In general, the control means may4be considered to represent a form of band-selection and noisediscrimination circuit capable of distinguishing the aforementionedrepetitive components from extraneous vibrations produced by movement ofthe car along the rail. The control means should 4be capable of making arelatively accurate determination of the car speed, from the frequencyof the repetitive components, so that the Control signal can lclose andopen the retarder to release the car at a given pre-set speed.

Other and further objects of the present invention will be apparent fromthe following description and claims and are illustrated in theaccompany drawings which, by way of illustration, show preferredembodiments of the present invention yand the principles thereof andwhat is now considered to be the best mode contemplated for applyingthese principles. Other embodiments of the invention embodying the sameor equivalent principles may be used and structural changes may be madeas desired by those skilled in the art without departing from thepresent invention and the purview of the appended claims.

In the drawings:

FIG. 1 is a schematic plan view and :block diagram of a railway carretarder including contr-ol apparatus comprising a speed-determiningsystem conslructed in accordance with one embodiment of the presentinvention;

FIG. 2 is a detail circuit diagram illustrating the initial circuits ofthe speed-determining system of FIG. 1;

FIG. 3 is -a detail circuit diagram showing gain-conirol circuits andthe principal speed-selection circuits used in the control system ofFIG. 1;

FIG. 4 is a detail circuit diagram of the principal noisediscriminationand control circuits incorporated in the system of FIG. l;

FIG. 5 illustraes the operating characteristics of an adjustablelow-pass tilter used in the system;

FIG. 6 is a block diagram, partly schematic, of another embodiment ofthe invention;

FIG. 7 is a schematic plan view and block diagram of another embodimentof the invention; and

FIGS. 8 and 9 are detail schematic views of certain circuits utilized inthe embodiment of FIG. 7.

FIG. 1 illustrates the application of one embodiment of the Ipresentinvention to the Control `of a railway car retarder 10. As shown in FIG.l, car retarder 10 may include a Ipair of traffic rails 11 and 12 alongwhich a car may roll in the direction indicated by the arrow A. A pairof car-retarding rails or brake shoes 13 are disposed immediaielyadjacent rail 11 in position t-o engage the opposite sides of a carwheel as it traverses this portion of the trafc rail. A similar pair ofretarder rails 14 may be disposed adjacent traic rail 12 in position toengage the sides of the car wheels on rail 12 as they traverse retarder10. Suitable means are provided for acuating retarder rails 13 and 14conjointly, this mechanism being generally indicated in FIG. 1 by theretarder actuating mechanism 1S. It is not necessary to use both sets ofretarder rails 13 and 14; one set may be employed, braking only thewheels on one side of a car. If only one set of retarder rails is used,it may -be desirable to use a guard rail along the other traic rail.

Retarder actuating mechanism 15 may be any suitable electrical,mechanical, pneumatic or hydraulic mechanism capable of moving retarderrails 13 and 14 between an open or released position and a closed orretarding position. When the retarder rails are in the open position, acar rolling along the track 11, 12 is permitted to pass through retarder10 without substantial braking. When the retarder rails are in theirclosed position, however, the rails engage the sides of the car wheelsand provide a substantial 'braking etIect on the rolling car.

In rearder 10, traiiic rail 12 may be of conventional construction.Traliic rail 11, however, is provided with a series of surfacediscontinuities at equally spaced intervals along the surface of therail. In the illustrated apparatus, these surface discontinuitiescomprise a series of shallow grooves 16 in the upper or bearing surfaceof rail 11, the grooves extending transversely of the rail surface. Byway of example, the grooves 16 in rail 11 may be of the order ofone-eighth to one-quarter inch wide; preferably, these grooves are madequite shallow in order to avoid any weakening of the rail. The spacing Sof the grooves or discontinuities 16 along the length of rail 11 is notparticularly critical, except that this spacing must be madesubstantially smaller than the circumference of a railway Wheel and alsomust be substantially smaller than the spacing -between any two adjacentwheels on a railway track. Typically, the spacing S between the grooves16 may be of the order of one to tive inches. Whatever the selectedspacing for the discontinuities or grooves 16, the spacing should bemaintained constant to alford an effective means for causing rail 11 tovibrate at frequencies representative of the speed of a car moving intoand through retarder 10.

It is not essential that the discontinuities 16 take the form `ofshallow grooves `in the upper surface of trafiiC rail. 11. Instead,these discontinuities may comprise relatively shallow notches or pitsformed in the upper surface of the rail or in the corner of the railadjacent `the wheel flange. Conversely, relatively small projectionscould be formed on the rail surface, since the purpose of the raildisconlinuities is to set up a vibration in the rail that isproportional to the speed of a railway car wheel moving along the rail.The grooved construction described hereinabove is preferred, however,because it affords the least problems with respect to machining of therail, wear of the rail, and maintenance of the requisite regular andconsistent series of surface discontinuities.

The speed-determining system of FIG. 1 also includes transducer meansfor generating an initial electrical signal that is representative ofvibration of the traiiic rail 11. This transducer means comprises a pairof individual sensing or pickup devices 17A and 17B that are mounted onthe base or web portion of traffic rail 11 between points 11A and 11Bdefining the limits of the serrated portion of the traffic rail. Avariety of ditcrent specific forms of vibration-sensitive pickup may beutilized for the devices 17A, 17B. Typically, these devices may compriseconventional velocity-actuated transducers each capable of producing anelectrical output signal directly representative of vibration of themember upon which they are mounted, the rail 11. Inasmuch as pickupdevices of this kind are Well known in the art and are commerciallyavailable, construction of the pickups is not described in detailherein.

The notched portion of rail 11 extends well ahead of the retarder rails13 and 14. In the illustrated arran-gement, the retarder rails are ofgiven length L and the notched section of rail 11 ahead of the retarderrails is of a corresponding length. Transducer 17A is 4located adistance D from the initial end 11A of the notched portion of rail 11.Transducer 17B is located a corresponding distance D from the outlet endof the retarder. This spacing is not critical, however, and is shownonly to afford a complete illustration of this embodiment of theinvention.

The transducers 17A and 17B are both electrically connected to a speedcontrol unit 18. The speed control unit 18 includes frequency-sensitivemeans for segregating, from the initial electrical signals from thetransducers 17A and 17B, speed-representative signals comprisingrepetitive components of the initial electrical signal produced byengagement of a car wheel with the surface discontinuities or grooves 16Iin the traffic rail 11. That is, speed control unit 18 provides for theeffective elimination of extraneous vibrations of notched rail 11, inthe signals from the transducer means 17A, 17B, such as might beproduced by a flat spot or spots on a vehicle wheel, by irregularitiesin the traffic rail 11 at some point other than the grooves 16, or byother similar sources of extraneous vibration. Speed control unit 1Salso includes means for utilizing the speed-representative signals todevelop a control signal, suitable for controlling operation of retarderactuating mechanism 15.

In control unit 18 the two pickups 17A and 17B are connected to anautomatic signal level switching circuit 21. Switching circuit 21 isemployed only to select the signal of maximum amplitude, from the twopickup devices, so that a single signal may be utilized to actuate theremaining circuits of control unit 18. It is not es.

sential that two pickup devices be employed; instead, a single pickupmay be utilized, in which case the automatic signal level switch 21 maybe eliminated. On the other hand, additional pickups may be included inthe system if desired. Selection of a single pickup from multipletransducers, in a given application, is dependent primarily upon thesensitivity of the transducers utilized and the damping characteristicsof traffic rail 11 with respect to vibrations caused by movement of acar wheel along the rail.

The output of switching circuit 21 is coupled to a cornpressor amplifier22 that functions primarily as an automatic gain control for the system.The compressor amplifier includes, in its output stage, a high-passfilter with a relatively low cut-off frequency, this filter beingemployed to eliminate, to a substantial extent, the effect ofverylow-frequency signals produced by flat spots or other.

irregularities in car wheels. The compressor amplifier Iis coupled, int-urn, to a clipper circuit 23 that is connected to an adjustablelow-pass filter 24. Adjustable filter 24 is the speed-selection deviceof the systemV and is employed to adjust the system to release cars fromretarder at varying speeds dependent upon the requirements of theclassification yard. It is essential that fil-ter 24 have a sharpcut-off characteristic in order to afford adequate differentiationbetween different desired output speeds.

The output of filter 24 is coupled to a driver amplifier 25 utilized toactuate a signal relay and control circuit 26. Circuit 26, in turn,actuates a brake relay circuit 27 that is electrically connected toretarder actuating mechanism to control the operation of the retarder.

In addition toV the connection to clipper circuit 23, the output ofcompressor amplifier 22 is coupled to a lowpass filter 28 that isconnected in series with a high-pass filter 29 to a noise amplifier andrelay circuit 31. Noise amplifier and relay circuit 31 is coupled tosignal relay circuit 26. A portion of the signal relay of circuit 26 isincorporated in a bypass circuit that shunts low-pass filter 2S forcertain operating conditions, as described ,more fully hereinafter.

In c-onsidering operation of the system illustrated in FIG. 1, it mayfirst be assumed that there is no car ,entering retarder 10. Retarderactuating mechanism 15 is set to maintain the retarder open. Since nocar is rolling over the notched traffic rail 11, the only output signalsfrom transducers 17A and 17B are extraneous noise signals of shortduration. These signals are not effective to actuate control system 18,so that the retarder remains in its open or released braking condition.

A car entering retarder 10 first engages the notched trafhc rail 11 atpoint 11A, the direction of car entrance being indicated by arrow A.Movement of the car wheels along the serrated tra'ic rail produces avibration in the traffic rail having frequency components determined bythe spacing of notches 16 and 'by the speed at which the car is moving,but with many harmonics. Vibration of the rail is sensed by transducers17A and 17B, producing initial electrical signals having repetitivecomponents at frequencies corresponding to the frequencies at which thetraffic rail .is vvibrated. When the car first enters the retarder, atpoint 11A, the signal produced by the first transducer 17A issubstantially stronger than that produced by the second pickup 17B,since the car is closer to transducer 17A and, accordingly, the dampingeffect of transmission of vibration along the rail is less with respectto the first pickup unit.

The output signal from transducer 17A, accordingly, is selected byswitch 21 and supplied to compressor amplier 22. The compressoramplifier and clipper 23 serve primarily to maintain a constantamplitude output despite substantial variations in the amplitude of thesignal received from the transducer. This automatic gain con- 5, troleffect is desirable because the amplitude of the output signal from thetransducer will vary to a substantial extent as the car moves aiongtraffic rail 11.

Filter 24 is set, in advance, to a given frequency determined by thedesired maximum speed for cars leaving the retarder. Assuming that thecar enters at a speed above the pre-set exit speed, the signal fromclipper 23 is composed primarily of frequencies higher than the settingof the adjustable low-pass filter 24. Consequently, the signal issubstantially attenuated and does not effect operation of the signalrelay and control circuit 26. The same signd, however, is supplied tonoise amplifier 31 through filter circuits 28 and 29. The signalsupplied to noise amplifier 31 actuates a relay, referred to herein asthe noise relay, completing an energizing circuit for the signal relayof unit 26.V Upon actuation of the signal relay, brake relay 27 isenergized -to complete an operating cir- Vcuit that supplies a controlsignal to retarder actuating mechanism 15. Accordingly, actuatingmechanism 15 is energized to close retarder 10.

Closing of the retarder is preferably accomplished before the railroadcar reaches retarder rails 13 and 14. It

lis for this reason that the serra-ted trafiic rail 11 is extended wellahead of the retarder mechanism, giving sufficient time to close theretarder before the car reaches braking position.

When the car is engaged by retarder rails 13 and 14, it is deceleratedat a rate depending upon the weight of Vthe car, its initial speed, thebraking force applied to the retarder rails by the retarder actuatingmechanism, and other factors common to railretarder systems. When thecar is decelerated to a speed below the exit velocity for which filter24 is adjusted, the filter applies a signal to driver amplifier 25,which in turn actuates circuit 26 to de-energize the signal relay. Whenthe signal relay drops out, the operating circuit for brake relay 27 isinterrupted, with the result that the control signal applied toactuating mechanism 15 is changed and mechanism 15 restores the retarderto its open or released position.

When signal relay 26 drops out, at the time that braking of the car iscompleted, the bypass circuit normally effective for low-pass filter 28in the noise channel of the system is opened, effectively placing thelow-pass filter in the signal channel to the noise amplifier 31. This isdone to prevent high order harmonic signals from holding the noise relayenergized at speeds below a predetermined minimum setting established asthe lower limit of retarder control.' In a typical system, this minimumsetting may be of the order of three miles per hour, with retardercontrol afforded over a range of three to fifteen miles per hour asdetermined by the setting of adjustable t filter 24.

As noted above, retarder 10 is opened or released while the car is stillmoving. Consequently, under ordinary circumstances the car continues itsmovement through the retarder, at its reduced speed, and eventuallypasses beyond the exit end 11B of notched rail 11. When this occurs, thesignal supplied to the control system 18 is l no longer adequate toenergize noise relay 31, with the result that the noise relay drops outand the system isready for a subsequent operation. The noise relay alsodrops out if the car speed drops below the minimum range of retarderoperation, as noted above, due to the operation of high-pass filter 29.65-

In some instances, if retarder 10 is established on a substantial grade,and extends for a substantial length along traffic rails 11 and 12, agiven car of free rolling characteristics may be braked to the desiredspeed but may accelerate substantially after the retarder has beenreleased and before the car leaves the retarder. If this should occur,the output signals from transducers 17A and 17B again increase infrequency above the frequency setting of adjustable low-pass filter 24.This `cuts off the actuating signal for driver amplifier 25 and permitsthe signal relay of circuit 26 to be energized again under control ofthe noise amplifier and relay circuit 31. Accordingly, -the retarder isagain actuated by the control signal supplied by the operation of thesignal relay and brake relay 27.

When a car first enters the retarder control system, at point 11A, thepredominant signal is that supplied by transducer 17A. Movement of thecar through the retarder, however, eventually results in production of asignal at transducer 17B that is of greater amplitude than the signalfrom the initial pickup 17A. When this happens, switch 21 is actuatedautomatically to transfer control of the system from the initialtransducer 17A to the second or outlet end transducer 17B.

FIG. 2 illustrates a specific circuit that may be utilized for theautomatic level switch 21 of FIG. 1. Switch circuit 21, as shown in FIG.2, comprises a first pre-amplifier 41 connected to pickup transducer 17Aby means of a coupling capacitor 42. Capacitor 42 is connected to thecenter terminal of a voltage divider comprising two resistors 43 and 44connected in series between a negative polarity unidirectional supply,designated as C-, and system ground. The coupling capacitor is alsoconnected -to the base electrode of a first transistor 45. The emitterof transistor 45 is connected to ground through a bias resistor 46. Thecollector electrode of transistor 45 is returned to the C- supplythrough a load resistor 47 and is connected to the base electrode of asecond transistor 48.

Transistor 48 is connected in an emitter follower circuit. The collectorof this transistor is connected directly to the C supply and the emitteris returned to ground through a load resistor 49. The output from thesecond stage of pre-amplifier 41 is taken `through a coupling capacitor51 connected to the emitter of the transistor.

The output stage of pre-amplifier 41 includes a first diode 52 connectedfrom capacitor 51 to ground and a second diode 53 connected in serieswith the capacitor to afford a half-wave voltage doubling rectiercircuit. The voltage doubler is completed by a capacitor 54 connectedfrom diode 53 to system ground. A resistor 55 connects the capacitor toan output terminal 56.

The second pickup transducer 17B is connected by a coupling capacitor 60to a pre-amplifier circuit 61 that is essentially identical with thepreamplifier 41. The only difference between circuits 41 and 61 is thatin the voltage doubler circuit comprising the output of amplifier 61 thetwo diodes 62 and 63 are reversed in polarity as compared with thecorresponding diodes 52 and 53 in the pre-amplifier for the first stageof the circuit. The voltage doubler circuit includes a capacitor 64, thevoltage doubler being connected to terminal 56 by means of a resistor65.

The common output terminal 56 of the two pre-amplifiers 41 and 61 isreturned to ground through an output resistor 67 which is connected inparallel with a capacitor 68. A resistor 69 connects terminal 56 to thecontrol electrode of a single-controlled silicon rectifier 71. Rectifier71 is connected in the energizing circuit for the operating coil 72 of alevel-switching relay 73. The cathode of rectifier 71 is returned toground through a self-biasing diode 74, the anode of the rectifier beingconnected to coil 72. The other end of coil 72 is connected directly toa suitable A.C. supply (not shown). A damping circuit, comprising adiode 75 connected in parallel with a capacitor 76, is connected acrosscoil 72 to protect rectifier 71 and relay 73 against transientover-voltage condit-ions.

Relay 73 includes a first movable contact 81 engageable with either oftwo fixed contacts 82 and 83, depending upon whether or not the relayhas ybeen actuated. A second set of contacts is incorporated in therelay and comprises a moving Contact 84 engageable with either of twofixed contacts 85 and 86. Movable contacts 81 and 84 are shown in thepositions maintained when relay 73 is de-energized.

Movable contact 81 comprises the principal output terminal for automaticswitch circuit 21 and is connected to compressor amplifier 22, asdescribed in detail hereinafter in connection with FIG. 3. Contact 82 ofrelay 73 is connected directly to the output of transducer 17B, whereascontact 83 is connected directly to the output of pickup device 17A.

Contacts 8486 of relay 73 are connected in a safety circuit, theoperation of which is described in substantial detail hereinafter.Contact 86 is directly connected by a line 87 to a noise amplifier (FIG.4). Contact 85 is connected through a resistor 88, in series with adiode 89, and by a conductor 91, to the noise relay that cornprises apart of the circuit 31 of FIG. 4. The movable contact of this portion ofthe relay is connected to a capacitor 92 that is returned to systemground.

Operation of the automatic level switch 21 of FIG. 2 is relativelysimple, and in effect, constitutes a comparison of the amplitudes of theoutput signals from transducers 17A and 17B to couple the signal havingthe greatest instantaneous amplitude to the compressor amplifier 22through movable contact 81 and the output conductor 93. Thus, when a carfirst enters the retarder section, engaging rail 11 at point 11A (FIG.1), the traffic rail is vibrated and an output signal yis produced byeach of transducers 17A and 17B. At first, the signal from transducer17A is substantially higher in amplitude than that from transducer 17B,due to the proximity of transducer 17A to the leading end 11A of theserrated traffic rail. Accordingly, when the two output signals fromamplifiers 41 and 61 (FIG. 2) are added together in opposite polarityacross resistor 67, the net signal across the resistor is of positivepolarity, the polarization of the voltage doubler 52-'54 in the outputof pre-amplifier 41. The resulting positive-polarity signal at terminal56 is supplied to the control electrode of rectifier 71 and triggers therectifier to conduction. With the rectifier 71 in its conductive state,relay 73 is energized, closing movable contact 81 on contact 83 anddirectly connecting transducer 17A through line 93 to the succeedingstage, compressor amplifier 22.

Subsequently, as the car passes through the retarder, a condition isreached in which the output signal from pickup 17B is of greateramplitude than that from transducer 17A. When this condition obtains,the summation signal appearing at terminal 56, and resulting from theaddition of the output signals from pre-ampliers 41 and 61 acrossresistor 67, swings negative in polarity. This is effective to cut offconduction through rectifier 71, with the result that relay 73 isde-energized. When the relay drops out, movable contact 81 returns toits normal position in engagement with contact 82, connecting thecompressor amplifier 22 directly to the output of transducer 17B.

In the illustrated circuit, it is preferable that capacitors 54 and 64be relatively large. The object in selecting capacitors of substantialsize is to obtain a charging time for the capacitors covering severalcycles at the fundamental frequencies of input signals within the normaloperating range of car speeds for the retarder system. That is, thecapacitors 54 and 64 establish a relatively long time constant for thisportion of the circuit. This prevents triggering of relay 73 in responseto short-duration, high-amplitude noise pulses. The relatively long timeconstant also serves to prevent chattering of the relay between itsenergized and de-energized conditions during time intervals in which theoutput signals from the two transducers are quite close in amplitude.

FIGS. 3 and 4, taken together, illustrate the remaining control circuitsof system 18. Inasmuch as the individual circuits in these two figuresare interconnected in several different instances, it is desirable toconsider the two figures conjointly, both in the discussion ofconstruction and in the description of operation.

FIG. 3 illustrates one construction that may be utilized for compressoramplifiers 22. In this circuit, the conductor 93 from signal levelswitch 21 (see FIG. 2) is returned to ground through a potentiometer101. The tap on potentiometer 101 is coupled by a capacitor 102 to thebase electrode of a transistor 103 in the first stage of the compressoramplifier. This first stage of the compressor amplifier is an emitterfollower circuit, the collector electrode of the transistor beingconnected through a de-coupling resistor 104 to the C- supply and theemitter being connected through a load resistor 105 to system ground.Resistor 104 is y'by-passed to ground by a de-coupling capacitor 110. Adegenerative feedback circuit comprising a capacitor 106 and a resistor107 connects the emitter of transistor 103 back to the base electrode.In addition, capacitor 106 is connected to the center terminal of avoltage divider comprising two resistors 108 and 109 connected fromsystem ground back through resistor 104 to the C- supply.

The output from the first stage of compressor amplifier 22 is takenthrough a capacitor 111 connected to the emitter of transistor 103, onone hand, and to the center terminal of a voltage divider comprising two.resistors 112 and 113 connected in series with resistor 104 from the C-supply to ground. A connecting resistor 114 is connected from capacitor111 to the base electrode of a transistor 115 in the second stage of thecompressor amplifier. The emitter of transistor 115 is returned tosystem ground. The collector of this transistor is connected through aresistor 116 in series with resistoi` 104 to the C- supply. The outputfrom transistor 115 is taken through a coupling capacitor 117 connectedfrom the collector electrode of the transistor to the base electrode ofa transistor 118 comprising the next stage of the amplifier. The baseelectrode of transistor 118 is also connected to a voltage dividercomprising two resistors 119 and 121 connected in lseries from the C-supply to system ground.

The emitter circuit for transistor 118 comprises a resistor 122 that isconnected in parallel with a capacitor 123 from the emitter to systemground. The collector circuit includes a load resistor 124 by means ofwhich the collector is connected t the C- supply. The output signal fromtransistor 118 is taken through a resistor 125 connected to thecollector and connected in series with a coupling capacitor 126 thatcomprises a part of a highpass filter 127. Resistor 125 is used to matchthe input of the filter to the output of the amplifier.

Filter 127 includes a further series capacitor 128 connected tocapacitor 126, the common terminal of the capacitors 126 and 128 beingreturned to ground through the series combination of an inductance coil129 and a capacitor 131. A further series capacitor 132 is connected tocapacitor 128, the common terminal of capacitors 128 and 132 beingreturned to ground through a resistor 133. Resistor 133 is the properload termination for the high pass filter 127. Resistors 125 and 133 areterminating resistors selected to make filter 127 give a fiat responseabove the cut-off frequency of the lter. Capacitor 132 couples filter127 to the base electrode of a transistor 134 that comprises the finalstage of compressor amplifier 22.

Transistor 134 is connected in a circuit similar to that for transistor103, this being an emitter follower utilized primarily to obtain a highinput impedance, to avoid loading the filter 127, but having arelatively low output impedance. The collector electrode of transistor134 is connected d-irectly to the C- supply. The emitter electrode isreturned to ground through two series-connected load resistors 135 and136 used to provide proper signal level to the clipper circuit 23. Toachieve the desired impedance characteristics, this stage of theamplifier includes a de-generative feedback circuit including a couplingcapacitor 137 connected in series with a resistor 138 from the emitterof transistor 134 to its base electrode. Capacitor 137 is also coupledto a voltage 10 divider comprising two resistors 141 and 142 connectedfrom the C- supply to ground for operating bias.

There are two output connections to transistor 134. The first outputconnection is taken directly from the emitter of the transistor, througha coupling capacitor 143 and a conductor 144 to the signal relay ofcircuit 26 and to the filter 28, as described more fully hereinafter inconnection with FIG. 4. The second output connection is taken from thecommon'terrnina-l of resistors and 136 in the emitter circuit, thisoutput extending to clipper circuit 23 and specifically to a couplingcapacitor 145 in the input of the clipper.

Compressor amplifier 22 is provided with a negative feedback circuitfrom the final stage, comprising transistors 134, back to the initialcompressor stage, transistor 115. This negative feedback circuitincludes a coupling capacit-or 146 that is connected to the emitter oftransistor 134 and to a series resistor 147. Resistor 147, in turn, isconnected to a half-wave voltage doubler circuit including a diode 148connected from the resistor to ground and a second diode 149 connected,from the resistor, in series with a capacitor 151, to ground. Diodes 148and 149 are oppositely polarized. The common terminal of diode 149 andcapacitor 151 is connected through two limiter diodes 153 and 154 and aresistor 155 to the base electrode of transistor 115 to complete thefeedback circuit. A fixed bias circuit is also provided for the baseelectrode of transistor l115, this fixed lbias circuit including -afilter circuit 157 connected by a conductor 158 to a low-voltage D.C.supply shown as a part of brake relay circuit 27 in FIG. 4.

Clipper circuit 23 is of conventional construction. It comprises aresistor 161 that is connected to the input coupling capacitor 145 andreturned to system ground through a pair of back-to-back diodes 162 and163. The output of the clipper comprises a coupling capacitor 164connected from the common termina-l of circuit elements 161-163 to thecenter terminal of a voltage divider including two resistors 165 and166, the voltage divider being connected from the C- supply to ground.The output terminal of the clipper is indicated by reference numeral167.

A preferred form of adju-stable low-pass filter 24 is shownA in FIG. 3.The first stage of this circuit comprises a pair of transistors 168 and169 each connected in an emitter follower configuration with a feedbackcircuit from the output of transistor 169 back to the input oftransistor 168, this feedback circuit including a twin-T filter 171.

Filter circuit 171 includes an adjustable input resistor 172, used forinput matching to give fiat response in the pass-back of the filter.Resistor 172 is connected to the output terminal 167 of clipper stage 23and is further connected in series with two adjustable resistors 173 and174 to the base electrode of transistor 168. A pair of capacitors 175and 176 are connected in series with each other across resistors 173 and174 and the common terminal of the two capacitors is connected throughan adjustable resistor 177 in series with a resistor 178 to the C-supply. Resistor 178 is the load resistor in the emitter circuit oftransistor 169.` The vfilter cir-cuit is completed by a capacitor 179connected from the common terminal of resistors 173 and 174 throughresistor 178 to the C- supply.

The collector electrode of transistor 168 is connected to system ground.The output circuit of the transistor comprises a Kload resistor 181connected in series with resistor 178 to the C- supply. The emitter oftransistor 168 is connected to the base electrode of transistor 169, thecollector electrode of transistor 169 being grounded.

The succeeding stage of filter 24 is of more conventional configuration.It comprises a pair of resistors 183 and 184 connected in series witheach other from the emitter of transistor 169 to the base electrode of atransistor 185. The common terminal of resistors 183 and 184 is returnedto the C- supply through an adjustable capacitor 186. The inputterminals to this stage, however, which is connected to the emitter oftransistor 169, is connected to a capacitor 187 that is not returneddirectly to the C- supply, instead being connected thereto through aresistor 188 that comprises the output resistor for the filter circuit24. This feed-back gives a sharper fall-off about the knee of thelow-pass filter characteristic.

Transistor 185 is connected in an emitter follower impedance matchingstage. The collector electrode is grounded and the emitter is connectedto the C- supply through a load resistor 189. The collector electrode oftransistor 185 is also connected through a low-pass filter 191 to thebase electrode of a transistor 192 in the output stage of circuit 24.Filter 191 may be essentially identical with the filter circuit in theinput to transistor 185, except that both capacitors are returneddirectly to the C- supply. Transistor 192 is again connected in anemitter follower impedance matching configuration with the collectorconnected to system ground and the emitter returned to the C- supplythrough load resistor 188. In addition to the feedback connection tocapacitor 187, the emitter of transistor 192 is connected by a conductor193 to the noise relay in circuit 31 (FIG. 4).

FIG. 4 shows the remaining control circuits of system 18. As showntherein, low-pass filter 28 is provided with an input terminal 201connected by the conductor 144 to the output stage of compressoramplifier 22 (FIG. 3). Low-pass filter 28 includes first and secondinductances 202 and 203 connected in series with each other from `inputterminal 201 to the filter to an output terminal 204. A capacitor 205 isconnected in parallel with coil 202. The common terminal of inductances202 and 203 is returned to system ground through a capacitor 206. Theinput and output terminals 201 and 204 are returned to ground throughtwo capacitors 207 and 208 respectively. In the output of the filter, aresistor 209 is connected from output terminal 204 to ground. Resistor209 matches the characteristic impedance of the filter to give a flatresponse in the pass-band.

High-pass filter 29 comprises an impedance-matching resistor 211connecting the filter to the output terminal 204 of low-pass filter 28.Resistor 211 is connected in series with two capacitors 212 and 213,capacitor 213 being returned to ground through a potentiometer 214 thatterminates the filter. The movable tap 215 on the potentiometerconstitutes the output terminal of the highpass filter. The commonterminal of capacitors 212 and 213 is returned to ground through aseries circuit comprising an inductance coil 216 and a capacitor 217.

A coupling capacitor 219 connects the output terminal 215 of high-passfilter 29 to the base electrode of a transistor 221 in the input stageof the noise amplifier and relay circuit 31. Transistor 221 is connectedin an emitter follower circuit similar to that provided for transistor103 in the input of compressor amplifier 22 (see FIG. 3) to afford arelatively high input impedance with a relatively low output impedance.Thus, the collector electrode of transistor 221 is connected to the C-supply. The emitter electrode is returned to ground through a loadlresistor 222. A 'de-generative feedback circuit is connected from theemitter to the base electrode, this feedback circuit comprising acapacitor 223 connected in series with a resistor 224. The commonterminal of capacitor 223 and resistor 224 is connected to the center ofa voltage divider comprising two resistors 225 and 226 connected fromthe C- supply to ground.

The output from the first or impedance-matching stage of noise amplifierand relay circuit 31 is taken from the emitter of transistor 221 througha coupling capacitor 227. Coupling capacitor 227 is connected to avoltage doubler half-wave rectifier comprising a diode 229 connected inseries with a capacitor 230 from capacitor 227 to ground, a furtherdiode 231 being connected in opposite polarization directly fromcapacitor 227 to ground. The output terminal 220 of the voltage doublercircuit is connected to resistor 232 that is returned to ground.Terminal 220 is also connected by conductor 87 to contact 86 of thesignal level relay 73 (FIG. 2). A gate current limiting lresistor 233 isconnected from terminal 220 to the control electrode of a signalcontrolled silicon rectifier 234.

Rectifier 234 is connected in the energizing circuit for the operatingcoil 235 of a noise relay 236. A self-biasing circuit comprising a diode237 is connected from the cathode of rectifier 234 to ground. A diode238 and a parallel capacitor 239 are connected across relay coil 235 toprotect the relay and diode 234. The remaining terminal of the relaycoil is connected to the secondary winding 240 of an input transformer250 in the brake relay circuit 27, the primary winding of transformer250 being connected to a suitable A.C. supply.

Noise relay 236 is provided with a first set of contacts comprising amovable contact 241 that engages a fixed contact 244 when the relay isde-energized but moves to engagement with a contact 243 when the relayis actuated. Contact 243 is connected to terminal 220 in the inputcircuit to control rectifier 234. Movable contact 241 is connected to acapacitor 242 that is returned to ground. Contact 244 is connectedthrough a coupling resistor 253 to a low-voltage D.C. supply circuitincluding a pair of resistors 254 and 255 connected as a voltagedivider, in series with a diode 256, across the secondary winding 240 oftransformer 250. The voltage divider is shunted by a capacitor 260. Itis thus seen that capacitor 242 is charged from the rectifier circuit,through resistor 253, during intervals in which the noise relay isde-energized, and may be discharged into the input circuit tosignalcontrolled rectifier 234 when the relay is actuated. Thisincreases the time constant of capacitor 230 and resistor 232 and tendsto lock in the relay once it has closed.

Relay 236 includes a second set of contacts comprising a movable contact246 engageable with a fixed contact 245 when the relay is de-energizedand movable into engagement with a second fixed contact 247 when therelay is actuated. Contact 247 is connected to the output of adjustablelow-pass filter 24 (FIG. 3) by means of conductor 193. Contact 246, themovable contact, is connected to a coupling capacitor 261 in the inputof driver amplifier 25. Contact 245 is connected to the movable tap of apotentiometer 262, shown as a part of brake relay circuit 27.Potentiometer 262 is connected across a pair of diodes 263 that areconnected in back-to-back relationship to afford a clipper circuit. Theclipper circuit is connected in series with a resistor 264 back to thesecondary winding 240 of transformer 250 to afford a fixed-voltage A.C.bias circuit.

Noise relay 236 includes one further set of contacts comprising amovable contact 251 that engages a fixed contact 248 when the relay istie-energized but moves into engagement with a second fixed contact 249when the relay is actuated. Contact 248 is connected, by conductor 91,to the contacts of switching device 73 in the automatic level switchcircuit 21 (FIG. 2). Movable contact 251 is connected to the A.C.supply, in this instance being connected to transformer secondary 240 inbrake relay circuit 27. Contact 249, on the other hand, is connected toone terminal of an operating coil 269 for a signal relay 271constituting a part of control circuit 26.

Considering next the driver amplifier circuit 25, it is seen that thecoupling capacitor 261 in the input to this circuit is connected to avoltage divider comprising two resistors 272 and 273 connected from theC- supply to ground. Capacitor 261 is also connected by a resistor 274to the base electrode of a transistor 275.

The emitter circuit of transistor 275 comprises a bias resistor 276 thatis returned to ground in parallel with a by pass capacitor 277. Theemitter is connected to the C- supply by a load resistor 278 and is alsoconnected to lboost circuits.

input of control circuit 26.

13 the base electrode of a second transistor 279 included in theamplifier.

Transistor 279 is connected as an emitter follower with the collectorconnected directly to the C- supply. kThe emitter is returned to groundthrough a load resistor 281. There is a feedback circuit from theemitter of transistor 279 to the base electrode of transistor 275, thisfeedback circuit including, in series, a resistor 282 and a capacitor283. The resulting circuit is essentially similar to circuits used inthe communication field for emphasizing lowfrequency components withrespect to high-frequencies, devices of -this'kind generally beingreferred to as bass- Thus, driver amplifier 25 affords greaterVamplification at relatively low frequencies than at high frequencies.

The output from driver amplifier 25 is taken through a resistor285connected to the emitter of transistor 279. Resistor 285 is connected inseries with a capacitor 286 that is coupled to a half-wave voltagedoubler rectifier circuit. The voltage doubler comprises a diode 287connectedfrom capacitor 286 to ground and a second diode 288 connectedwith opposite polarization and in series with a capacitor 289 fromcapacitor 286 to ground. The common terminal of diode 288 and capacitor289 is identified by reference numeral 291.

Voltagey doubler circuit 287-289 is shown as a part of Ithe signal relayand control circuit 26. A potentiometer 292 is connected in parallelwith the capacitor 289 of the voltage doubler circuit. The movable tapof the po- Stentiome'ter is connected through a resistor 293 to thelcontrol electrode of a signal-controlled `silicon rectifier 294'."Diode 294 is connected in series with the operating coil 269 of signalrelay 271.

There is another circuit connection to the control electrode oflrectifier 294, this second circuit connection being utilizedto afford afixed bias circuit for the control electrode. yThe bias circuit includesa current limiting resistor296 and a b ias resistor297 connected inseries with each other from the control electrode of rectifier 294 backto diode 256l in brake relay circuit 27. A`Zener diode '298 is connectedfrom the common terminal of resistors 296 and 297 to groundv to completethebias circuit.

, Signal relay 271 is provided with three sets of operating contacts.The first set comprises a movable conn tact 301 that engages, a fixedcontact 304 when the relay is de-venergized but moves to engagement witha second fixed contact 303 when the relay is actuated. Contact 3 01 isreturnedto ground through a capacitor 315. Conoutput terminal 2 91 ofthe voltage Adoubler circuit in the The next set of contacts in relay271 comprises a movj able contact 306 that engagesra fixed Contact 305when the relay is not energizedl butengages a second fixed contact 307lwhen the relay is actuated. Contact 305 is open-circuited. Contact 306is connected to output teryminal 204 of low-pass filter 28. Contact 307is connected'to the input terminal 201 of the low-pass filter.

Thus, itcan'be seen that when' signal relay 27,1,is enerand 311 of relay271 to afford a means for actuating the system to close the retarder ona manually controlled basis'.

Brake relay 27 may be connected to retarder actuating mechanism 15 inany one of a variety of ways, depending upon the construction of theretarder operating mechanism and the kind of control system required forits operation. In the arrangement shown in FIG. 4, the control signalconnection to the retarder operating mechanism is made through a singleset of contacts 325 of the brake relay. However, this connection may beconsiderably more complex if it is necessary or desirable to open orclose more than one circuit for the retarder actuating mechanism to putthe brakes on or off.

In addition, 'the brake relay may be provided with a second set ofcontacts 326 and appropriate indicator lights 327 and 328 to showwhether the brake relay has been actuated.

To facilitate description of the operation of the control system ofFIGS. 2-4, it is desirable first to consider typical examples of notchspacing and other factors affecting system operation. For example, thenotched spacing S (FIG. 1) may be established at 1`. 5 inches, therebyestablishing an approximate frequency-to-speed relation of elevencycles'per second per mile per hour. It will be understood, of course,that this particular notch spacing is given only by way of example andthat other spacing may be selected with consequent revisions in theoperating circuits.l In a typical installation, the length L isthirty-nine feet, the distance D fifteen feet (FIG. 1).

FurtherV to establish the operating characteristics for the system aslshown in FIGS. 2-4,` it may be assumed that'the compressor amplifier 22is provided with an output filter 127 affording an attenuation of threedecibels at twenty cycles per second, a frequency representative ofapproximately two miles per hour, with virtually infinite rejection -forfrequencies below fourteen cycles per second. Low-pass filter 28 may beconstructed to have a cut-off'frequency of approximately one hundredsixty-five cycles per second, corresponding to about fifteen miles perhour with the aforementioned notched spacing of 1.5

As noted above, the lower limit of the operating' range for the systemis determined primarily by the construction *of high-pass filter 29. Inthe following operational description, it is assumed that this filter isconstructed to have a cut-offA frequency of about thirty-.three cyclesper second, corresponding to a lower speed limit, for retarder controlof three miles per'hour'.

The exit:` speed for the car is established, as noted above, byadjustment of low-pass filter 24. In the' followther amplified by driveramplifier 25.

Y Initially, before a car enters the immediate area. of the retarder,all ofthe operating relays of the system are deenergized. All relays areillustrated in thiscondition.

ySwitching relay 73 in the automatic level switching circuit 21 (FIG. 2)is held de-energized by the self-biasing circuit for diode 71,comprising diode 74, which maintains diode 71 in cut-off condition.Similarly, noise relay 236 is maintained de-energized by the smallself-bias afforded by diode 237 (FIG. 4). Signal relay 271 isvmaintained def-energized by the ope'n circuit win the operating circuitfor coil 269 thatY appears at contacts 249 and 251 of the noise relay.Brake relay 27, on the other hand, is held in its unactuated conditiondue to the fact that operating coil 321'is open-circuited at contacts309 and 311 of signal relay 271.

`With operating conditions as described above, a car 'may enter theretarder control system at point 11A(FIG.

1) at a speed substantially above the setting of adjustable low-passfilter 24. The entering car, as it passes over notches 16, vibrates rail11 at a fundamental frequency determined by the car speed. Thus, with anotch spacing S of 1.5 inches, a car entering the retarder at six milesper hour vibrates the rail at a fundamental frequency of approximatelysixty-six cycles per second. A car travelling at eight miles per hourwould vibrate the rail at a fundamental frequency of eighty-eight cyclesand a car travelling twelve miles per hour would produce a vibrationhaving a fundamental frequency of one hundred thirty-two cycles persecond. However, vibration of the rail 11 is not limited to thefundamental frequency by any means. Rather, the rail also vibrates atfrequencies representative of multiple harmonics of the fundamentalConsequently, the vibration of the rail is a quite complex phenomenon.

Initially, with an over-speed car entering the retarder system, relay 73is actuated, as described above, to connect only transducer 17A tocompressor amplifier 22. The compressor amplifier amplifies the signalfrom transducer 17A, producing an -output signal that is maintainedwithin a limited amplitude range despite substantial variations in theamplitude of the input from the pickup device. It should 'be noted, inthis regard, that diodes 153 and 154 in the negative feedback circuit ofamplifier 22 are conductive only when the output signal exceeds one.volt.

The particular circuit illustrated in FIG. 3 for compressor amplifier 22provides an output signal amplitude variation limited to approximatelytwelve decibels, with an input variation of forty decibels. At the sametime, amplifier 22 affords positive gain, the output signal for aone-half volt input being approximately two volts R.M.S. The workingrange of compressor amplifier 22, with respect to input signalamplitude, is approximately 1.5 volts to 100 millivolts.

Not all of the input signal is effectively translated through thecompressor amplifier. Filter 127 effectively attenuates verylow-frequency components in the signal, these being componentsrepresentative of car speeds below approximately two miles per hour.

The output signal from compressor amplifier 22 is supplied, through thecircuit comprising coupling Capacitor 143 and conductor 144, to theinput terminal 201 of lowpass filter 28 (see FIG. 4). The repetitivecomponents of the signal below the cut-off frequency of filter 28, inthis instance below one hundred sixty-five cycles (l m.p.h.) .aretranslated to the output terminal 204 of the filter and, accordingly,supplied to high-pass filter 29. Filter 29 attenuates the frequencycomponents below approximately thirty cycles (3 m.p.h.) and supplies theremaining signal components to noise amplifier 31 through the connectionafforded by coupling capacitor 219. This signal is supplied to thecontrol electrode of rectifier 234, rendering rectifier 234 conductiveand completing an energizing circuit for noise relay 236.

As soon as noise relay 236 is actuated, contact 241 closes upon contact243, connecting capacitor 242 to the input terminal 220 of the controlcircuit for rectier 234. Capacitor 242, which has previously beencharged through its connection to the D.C. supply circuit comprisingrectifier 256, discharges, maintaining the noise relay 236 energized fora short time interval. This momentary holding circuit arrangement isutilized to make sure that noise relay 236 is energized; it alsoincreases the time constant of the circuit to guard against drop-out ofrelay 236 on momentary interrupting of the incoming signal.

Actuation of the noise relay also causes contacts 245 and 246 to openand close contacts 246 and 247. Opening of contacts 245 and 246interrupts the A.C. bias signal previously supplied to driver amplifier25 fr-om clipper circuit 262, 263. The ybias circuit keeps capacitor 289partially charged in the absence of pick-up signal. Thus, a delay Yininitial brake appljatin gan be effected by adjustment of potentiometer262. Also, momentary dropouts of noise relay 236 due to loss of signalwill not allow the brake to be re-applied when signal again act-uatesrelay 236 if there is some delay set in by this bias. Closing ofcontacts 246 and 247 completes an operating circuit to the input ofdriver amplifier 25 from the output circuit of filter 24 through theconnecting conductor 193 (FIGS. 3 and 4). At this point, however, nosignificant signal is supplied to the driver amplifier from filter 24,since the available signal is well above the cut-off frequency to whichthe low-pass filter has lbeen affect actuation of signal relay 271 atthis point.

Actuation of the noise relay, in response to signals indicative ofmovement of the over-speed car into the retarder control system, alsocloses contacts 249 and 251. Closing of these contacts completes anoperating circuit for the coil 269 of signal relay 271. The controlledrectifier 294 in the operating circuit of this relay is normally biasedtoward conduction by the circuit cornprising Zener diode 298.Consequently, the signal relay is actuated in response to actuation ofnoise relay 236.

When signal relay 271 is actuated, contacts 309 and 311 close. Thisestablishes an energizing circuit for the operating coil 321 of brakerelay 27. Thus, the brake relay is actuated and supplies a controlsignal to mechanism 15 that causes retarder 10 (FIG. 1) to be actuatedto its braking condition.

Actuation of signal relay 271 also closes contacts 301 and 303. Thisconnects capacitor 315 to the charging circuit 316, 317, charging thecapacitor for a purpose described hereinafter. Furthermore, contacts 306and 307 are closed, shorting out low-pass filter 28. Thus, as long assignal relay 271 remains actuated, and the brakes are on, high harmonicspresent in the output signal from compressor amplifier 22 are passeddirectly to noise amplifier circuit 31 to hold noise relay 236 in itsenergized condition. These high frequencies are more easily transmitteddown the track than lower frequency signals, a consideration that isespecially important in turning the brake on initially.

Retarder 10 is actuated, lby mechanism 15, to its closed or brakingcondition before the incoming car reaches retarder rails 13 and 14 (FIG.1). Hence, when the over-speed car reaches the retarder rails it isdecelerated at a rate dependent upon the friction characteristics of therails and the wheels engaged thereby and also dependent upon the forceapplied to the retarder rails by mechanism 15.

As long as the car speed remains above the setting of adjustablelow-pass filter 24, circuit conditions are not changed from thosedescribed immediately hereinabove. When the car has been decelerated tothe release speed established by the setting of filter 24, however, asignal of substantial amplitude is translated through the filter and issupplied to driver amplifier 2S through the circuit comprising conductor193, nose relay contacts 246 and 247, and coupling capacitor 261 (FIG.4). This signal is amplified in circuit 25, which emphasizes thelow-frequency components thereof as noted hereinabove. The

output signal from the driver amplifier is rectified in the voltagedoubler circuit 287-289 and is supplied to the control electrode ofrectifier 294 through the circuit comprising potentiometer 292 andcurrent-limiting resistor 293. As a consequence, rectifier 294 is drivento cut-off, de-energizing signal relay 271.

When signal relay 271 drops out, contact 301 returns to engagement withcontact 304. This change is effective to interrupt the charging `circleto capacitor 31S and connects the capacitor to terminal 291. Thus,capacitor 315 is discharged through potentiometer 292, helping tomaintain the signal relay in de-energized condition. It also increasesthe circuit time constant about 5:1 so that subsequent signal drop-outswill not re-apply the brake. Also, it delays re-application of thebrakes in case the car speeds up; hence the retarder doesnt chatter onand off if speed is near the filter cut-off frequency. As it isdischarged, capacitor 315 charges capacitor 289 more negatively. Theillustrated circuit arrangement gives rapid response to changes insignal condition in the output of drive amplifier 25, yet is effectiveto prevent premature reapplication of the brake on slow-speed cars.

Of course, as soon as signal relay 271 drops out, contacts 309 and 311open. This interrupts the operating circuit for brake relay coil 321 andthe brakes go ofi.

Contacts 306 and 307 are opened, when signal relay 271 drops out,removing the shunt across low-pass ilter 28. That is, filter 28 iseectively re-connected in the input circuit of noise amplifier 31whenever the signal relay is de-energized and the brakes are off. Thelowpass lilter prevents high order harmonics of the repetitive vibrationsignals from keeping noise relay 236 actuated after the car speed dropsbelow the minimum value established by high-pass filter 29.

If, in the course of braking, the speed of the car moving through theretarder drops below the cut-off speed established by high-pass filter29, noise relay 236 usually drops out, although harmonic signals maykeep the noise relay energized for a short interval. In any event, whenthe car clears the end 11B of the retarder, the input signal to thenoise amplifier is insufficient to maintain the noise relay energized.Thus, the noise relay drops out and the system is ready for the nextoperation.

In some instances, a car entering the retarder may be travelling at aspeed below the speed for which adjustable low-pass t'iiter 24 is set.If this occurs, the sequence of operations is essentially as describedabove except that lter 24 immediately passes a signal of substantialamplitude. As a consequence, a de-energizing signal is immediatelyavailable, from driver amplilier 25, operating to cut oft signal relay271. Thus, although noise relay 236 is actuated, signal relay 271remains effectively opencircuited at the signal-controlled rectifier294. Since the signal relay cannot be actuated, the brakes are notapplied and the car rolls through the retarder without braking.

As each car moves along rail 11, and into the retarder, it causesswitching circuit 21 to actuate relay 73 to connect first pickup 17A andsubsequently pickup 17B to the remaining circuits of the control system.In addition to switching the input connections to compressor amplifier22, relay 73 controls a safety circuit, connected to contacts 84-86,that protects the system against cars approaching the retarder at veryhigh speeds.

With reference to FIGS. 2 and 4, it can be seen that capacitor 92 isinitially charged through resistor 88 and diode 91 by means of theconnection extending back through contacts 248 and 251 of the noiserelay to the A.C. supply. The contacts 84 and 85 of relay 73 are inseries in this charging circuit.

When a car enters the retarder at point 11A, actuating relay 73, thecharging circuit connection to capacitor 92 is interrupted. At the sametime, the capacitor is effectively connected, through contacts 84 and 86and conductor 87, to the input terminal 220 for the signal-controlledrectifier 234 of the noise amplifier and relay circuit 31. Discharge ofcapacitor 92 drives rectifier 234 conductive. As a consequence, noiserelay 236 is energized long enough to actuate signal relay 271 andestablish a momentary shunt for low-pass filter 28. Otherwise, if areally high speed car were coming in, the speed of the car being overthe cut-off for low-pass filter 28, the filter might prevent transfer ofan adequate energizing signal to noise amplifier and relay circuit 31with the result that the brakes would not be applied. On the other hand,in a given installation it may be desirable to construct the retardercontrol deliberately to pass any car at speeds over, for example,fifteen miles per hour, in which case the safety circuit describedimmediately above may be omitted. As long as relay 236 remainsenergized,

18 capacitor 92 cannot be charged, and further operations of relay 73have no undesirable effects.

In order to afford a complete illustration of the invention, circuitparameters for the circuits of FIGS. 2-4 are listed hereinafter. Itshould be understood that these data are set forth only by way ofexample and not as a limitation on the invention.

Resistors and potentiometers 43 kilohms 180 44 do 18 46, 113, 147 do 147, 55, 65, 278 do 4.7 49, 161, 297 do 2.2 67, 165, 166, 189, 232 do 4769, 233 do 27 88, 101, 107, 133, 138, 184, 214, 224, 293,

296 ki1ohms 100 105, 116, 124, 183, 188, 178, 222 do l0 108, 109, 141,142, 225, 226 do 68 112 do 120 114, 209, 274 do 2.7 104 ohms 680 119,272 kilohms 150 121 do 3.9 122, 276 ohms 470 125, 209, 282 kilohms 56135, 136 do 1.5 172 do 82-200 173, 174 do 8.1-27 177 do 1.8-13.3 253 do6.8 273, 317 do 8.2 281 do 1.8 285 do 1.2 292 do 33-83 316 do 39Capacitors Microfarads Inductors Henries Transistors, diodes,rectifz'ers 45, 118, 275 2N383 48, 134, 279 2N586 52, 53, 229, 231 1N56A71, 234, 294 3A15A 74 S1010 103, 115, 221 2N1309 148, 149, 153, 154,162, 163, 237, 287, 288 1N456 168, 169, 185, 192 T1495 298 1N1507 19Operating voltages Transformer secondary 240, volts A.C 6 C-, volts D.C.22.5

In considering the basic characteristics of speed control system 18, asdescribed in detail hereinabove, it should be noted that automaticsignal level switch 21 serves primarily to select the maximum amplitudesignal from transducers 17A and 17B and is not necessary to the systemif only a single transducer is employed or if other means are affordedto accommodate multiple transducers as in the system of FIG. 7,described hereinafter. Compressor amplifier 22 serves primarily toreduce the initial signal from the transducers to a limited amplituderange suitable for further use in the control system, the AGC effectbeing further assisted by clipper 23 before the signal is passed on tolow-pass filter 24.

The high-pass filter 127 in the output of compressor amplifier 22 (FIG.3) is not essential to operation of the system, since it servesprimarily to set a lower frequency limit for signals passed on to otherportions of the control circuit. However, this high-pass filter isdesirable because it affords positive protection against erroneousactuation of the retarder as the result of low-frequency vibration ofrail 11 that would not be attributable to the movement of a railwayvehicle along the rail. An example of a source of such spuriouslow-frequency signals would be the operation of a spike driver somewherealong the track, engagement of car wheels with track joints ahead of theserrated rail, and fiat spots on the car wheels, any of which canproduce high-amplitude low-frequency vibration signals along the track.All of these troubles are most noticeable before a car reaches thenotched rail. Once the car is on notches 16 the signal from the notchesoverrides everything else. Filter 127 also helps speed up the responseof the AGC circuit, compressor amplifier 22.

The most important control element in system 18 is adjustable low-passfilter 24; since this device establishes the release speed for the cars,it must have a sharp cut-off characteristic and must be adjustable toany release speed desired for the system. The output signal from filter24 may aptly be termed a release signal, and it is this release signalthat is supplied to driver amplifier 25 to deenergize the signal relay271 of circuit 26 and actuate relay 27 to release the retarder. Inconnection with driver amplifier 25, the low-frequency emphasis affordedby the operating characteristic of this amplifier is quite desirablebecause it is essential to release retarder as soon as the car is slowedto the desired exit speed. There are always some losses at lowfrequencies, due to the velocity-dependent pick-ups. Also, at lowfrequencies the periods of signal drop-out are longer, due to the longertime it takes the car to get from one pick-up to the other, andcapacitors 289-315 must be charged to a higher level to maintain controlduring such drop-outs.

Control of signal relay 271 of circuit 26 is actually effected bysignals supplied through two different channels, as will be apparentfrom the foregoing description. Thus, initial control of the relay iseffected by a first channel comprising filters 28 and 29 and the noiseamplifier and relay circuit 31. The signal supplied to circuit 26through this channel tends to actuate the signal relay to its actuatedor brake-applying condition in response to both low and high frequencysignals, the operating range of the signal channel being established bythe filters 28 and 29. The control effected through this channel, hovever, is over-ridden by the signal supplied to circuit 26 through thealternate channel comprising clipper 23, lowpass filter 24, and driveramplifier 25, whenever the speed of a car is reduced to a level belowthe speed setting of filter 24. In this connection, noise relay 236functions as an auxiliary switching device with respect to signal relay271; the two relays cooperate to afford the necessary control function.

The operating circuits shown in FIGS. 2-4 can, of course, be modifiedsubstantially without departing from the present invention. By way ofexample, it should be noted that it is not essential to use relays asthe principal control elements in circuits 26, 27 and 31. Instead, othersignal-actuated control devices, preferably switching devices, may beemployed in all of these circuits. Changes of a similar nature can bemade in Virtually all of the individual operating circuits.

In sorne installations, it is possible to utilize a retarder 10 that isnormally maintained in a closed or braking condition. Where theinstallation permits operation on this basis, system 18 may besimplified substantially by eliminating the signal channel comprisingfilters 28 and 29 and noise circuit 31. Control of the retarder is theneffected oniy by the speed-control channel comprising clipper 23, filter24, and driver amplifier 25. However, a system of this kind, if set fora low release speed, could operate to lock a car in a retarder, acondition that would not be desirable in most installations because ofthe possibility of damage from a second car entering the retarder. Thesignal channel comprising circuits 23, 29 and 31 prevents thiscompletely; in effect, this signal channel controls the entire systemand is effective to release the brakes when a car is below the minimumspeed for which high-pass filter 29 is constructed (in this instance 3m.p.h.) even if the main speed control comprising the channel includingfilter 24 fails to operate.

As noted hereinabove, one of the most critical elements of the controlsystem is the adjustable low pass filter 24. FIG. 5 illustratesoperating characteristics for circuit 24, and specifically for theparticular circuit arrangement illustrated in FIG. 3, at three differentspeed settings. The initial curve 561 in FIG. 5, a plot of the outputsignal amplitude of the filter in volts as a function of frequency incycles per second, shows a three mile per hour setting of the low passfilter. As can be seen from curve 501, the amplitude of the signaloutput from the filter remains substantially constant until thefrequency reaches approximately thirty cycles, at which point the signalamplitude begins to drop o. The output signal is down three decibels ata frequency of approximately 33.8 cycles per second, correspondingsubstantially to a car speed of three miles per hour.

The second curve 562 in FIG. 5 illustrates the operation of filtercircuit 24 when adjusted for a speed of four miles per hour. In thisinstance, the signal is down three db for a frequency of approximatelyforty-six cycles per second. The third curve 503 in this figure pertainsto a filter setting for seven miles per hour, the signal beingattenuated approximately three db at a frequency of about 81.5 cyclesper second. In each instance, the output signal employed drops off verysharply after the cutoff frequency is reached, a characteristic that isnecessary in the low pass filter if the system is to function properly.

FIG. 6 illustrates a speed-sensitive control system 368 for a railwaycar retarder that comprises a second embodiment of the presentinvention. In many respects, system 368 is substantially similar to thecontrol system described in detail hereinabove. Thus, it comprises thetwo pickups 17A and 17B that are mounted at suitable positions along anotched or serrated rail in the same manner as in the embodiment ofFIG. 1. Pickup devices 17A and 17B are again connected to an automaticsignal level switch 21 that may be constructed in the same manner asillustrated in detail in FIG. 2.

The output of switching circuit 21 is again coupled to a compressoramplifier 22 that .is in turn coupled to a clipper circuit 23. Theoutput of the clipper is coupled to the adjustable low pass filter 24,the low pass filter again being connected to a driver amplifier 25.Circuits 22-25 may be essentially similar in construction to thecorresponding circuits described in detail hereinafter in connectionwith FIGS. 3 and 4.

The output of driver amplifier 25 is connected to a rectifier circuit369 that may be essentially similar to the rectifier circuit 287, 288 inthe input of circuit 26, FIG. 4. Rectifier 369 is connected to a signalrelay control circuit 370 that may be essentially similar inconstruction to the signal controlled rectifier circuit shown in FIG. 4in connection with relay 271. As before, a capacitor 289 is incorporatedin the signal relay control circuit, being returned to ground.

Circuit 370 controls the actuation of a signal relay 371 having anoperating coil 372. Relay 371 comprises three sets of contacts. Thefirst set of contacts includes a movable contact 406 that normallyengages a first fixed contact 405 but that is engageable with a secondfixed contact 407 upon actuation of the relay. Another set of contactsfor relay 371 includes fixed contacts 408 and 409. When the relay isde-energized contact 408 is engaged by a movable contact 411; when therelay is actuated, contact 411 moves into engagement with contact 409. Afurther set of relay contacts comprises a movable contact 401 that isordinarily engaged with a fixed contact 404 but that engages a secondfixed contact 403 when the relay is actuated.

Contact 405 of signal relay 371 is connected to the output of compressoramplifier 22. Movable contact 406 is connected to the input of a highpass filter 29, which may be constructed to correspond substantially tothe filter circuit 29 illustrated in FIG. 4. Contact 407, on the otherhand, is connected to the output of the adjustable low pass filter 24.Thus, actuation of signal control relay 371 is effective to change theinput circuit to high pass filter 29 by incorporating low pass filter 24in this circuit as described more fully hereinafter.

The movable contact 401 in the second set of relay contacts is returnedto ground through a capacitor 415. The fixed contact 404 in this set isconnected to a low voltage D.C. supply B-jthrough a resistor 412.Terminal 403 is connected to capacitor 289 in the coupling circuitbetween rectifier 369 and control circuit 370.

In the final set of contacts for relay 371, fixed contact 409 is leftopen circuited. Contact 411 is grounded and contact 408 is utilized toafford a signal connection to the noise relay of the embodiment, asdescribed more fully hereinafter.

High pass filter 29 is connected yto a noise amplifier circuit 421 thatmay be essentially similar in construction to the initial stage ofcircuit 31 in FIG. 4. In the system of FIG. 6, it may be desirable toprovide for additional amplification in the noise channel, in which casea further amplifier circuit may be incorporated in the signal channel,preferably ahead of filter 29. The output of noise amplifier 421 isconnected to a rectifier circuit 422 that may be essentially similar tothe voltage doubler 229, 231 in the circuit 31 of the initiallydescribed embodiment. Again, the rectifier is connected to a controlcircuit 423 that may constitute a signal-controlled rectifier, as in therelay control circuits described hereinbefore, the control arrangementcomprising circuits 422 and 432 including the capacitor 230.

Circuit 423 controls actuation of anoise relay 436 having an operatingcoil 435 and two sets of contacts. The first set of contacts in thisrelay includes a movable contact 441 that is normally engaged with afixed contact 444. Contact 441 is moved to engagement with a secondfixed contact 443 upon actuation of the relay. Movable contact 441 isreturned to ground through a capacitor 442. Contact 444 is connected,through a resistor 453, to the B+ supply. Contact 443 in this set isconnected back to capacitor 230 in the control circuit for the noiserelay.

The second set of contacts for relay 436 comprises a movable contact 446that is normally engaged with an open-circuited contact 445. Theremaining contact 447 in this set, which is engaged by contact 446 whenthe relay is actuated, is connected to one terminal of the operatingcoil 321 of the brake relay 27. The other terminal of coil 321 isconnected to the AC supply. As before, the operating contacts 325 of thebrake relay are connected to a retarder actuating mechanism 15 asdescribed hereinabove in connection with FIG. 1. The movable contact 446in this portion of relay 436 is connected to contact 40S of the signalrelay 371.

Initially, and prior to the time `that a car enters the immediate areaof the retarder, the operating relays of system 363 are de-energized andare in the positions shown in FIG. 6. When a railway car enters theretarder control system at point 11A (FIG. l) it vibrates the trafc railat a fundamental frequency determined by the speed of the car. If thefundamental signal frequency is above the setting of adjustable low passfilter 24, as would be the case with a car entering above the desiredrelease speed set for the retarder, the output signal from low passfilter 24 is quite weak and is insufficient to actuate signal relay 371.The output signal from the compressor amplifier 22, however, is ofsubstantial amplitude; it should be remembered that circuit 22 affords asubstantial AGC action and produces a signal of usable amplitude over awide range of frequencies. This signal is passed through filter 29,which again affords attenua-tion only at frequencies below approximatelythirty cycles. The signal is arnplied (circuit 421), rectified (circuit422) and applied to control circuit 423 to energize noise relay 436.

When relay 436 is actuated, contacts 446 and 447 close to complete anoperating circuit for the coil 321 of brake relay 27. Consequently,contacts 325 are actuated and produce an output signal that is suppliedto mechanism 15 to establish the retarder in its braking condition.

With the retarder in braking condition, the car is decelerated asbefore. When the car speed is reduced to a level at which the engagementof the car wheels With the notches in serrated rail 11 occurs at afrequency beloW the setting of filter 24, an output signal ofappreciable arnplitude is available from the low pass filter. Thissignal is supplied to amplifier 25, is rectified in circuit 369, and isapplied to circuit 370 to actuate signal relay 371. AS soon as relay 371is actuated contacts 408 and 411 open, interrupting the energizingcircuit for brake relay 27 and restoring the retarder to its releasedcondition.

Actuation of signal relay 371 also affects the input to high pass filter29. Thus, when the signal relay is actuated, contacts 405 and 406 open,interrupting the initial input circuit to the high pass filter. Contacts406 and 407, however, are now closed. Accordingly, high pass filter 29is now provided with an input circuit that includes, in seriestherewith, the adjustable low-.pass filter 24.

By incorporating low-pass filter 24 in series in the input circuit tohigh-pass filter 29, upon actuation of signal relay 371, the adjustablelow-pass filter is made to perform the basic function of the low-passfilter 28 in the embodiment of FIG. l. Thus, the additional low-passfilter incorporated in the noise signal channel of the initialembodirnent is eliminated, but its basic function is retained. Thepresence of low-pass filter 24 in the noise signal channel, after thebrakes have been released but with a car still rolling through theretarder system, prevents high order harmonics of the repetitivevibration signals from keeping the noise relay actuated after the carspeed drops below the minimum value established by high pass filter 29.When the car speed drops below the speed represented by the cut-offfrequency of high-pass filter 29,

rnoise relay 436 is de-energized and the relay drops out.

Thus, the system is conditioned for the next operation as soon as thecar passes beyond the end of the notched rail and the actuating signalfor relay 371 is no longer available.

The safety circuit utilized in the initial embodiment is not required insystem 36S of FIG. 6. Thus, there is no low-pass filter in the noisechannel unless and until signal relay 371 is actuated. Consequently, acar travelling at very high speeds will :always actuate noise relay 436and establish the retarder in braking condition as soon as anappreciable signal is available from pick-ups 17A and 17B.

When signal relay 371 is in its normal tie-energized condition capacitor415 is charged through the charging circuit comprising resistor 412 andrelay contacts 401 and 404. When the signal relay is actuated by asignal indicative of a car moving at a speed below the setting of filter24, capacitor 415 is disconnected from its charging circuit and isconnected to capacitor 289 in the -control circuit for the signal relay.Thus, capacitor 415 fulfills the same basic functions as capacitor 315in the first-described embodiment; in this instance, however, the chargeon the capacitor is reversed in polarity because the object is tomaintain relay 371 in energized condition in order to release theretarder, the energized and de-energized operating conditions for thesignal relay being reversed as compared with the initial embodiment ofthe invention. As before, the connection of capacitor 415 to the controlcircuit rfor the signal relay also increases the control circuit timeconstant to avoid premature reapplication of the brake.

Capacitor 442, on the other hand, carries out the same basic functionsas capacitor 242 in the first embodiment of the invention (see FIG. 4).Capacitor 442 is normally connected to a charging circuit comprisingresistor 453 and contacts 441 and 444 for noise relay 436. When thenoise relay is actuated in response to movement of a car into theretarder, the capacitor is disconnected from its charging circuit and isconnected to capacitor 230 in the control circuit for relay 436. Ascapacitor 442 discharges, it is effective to maintain the noise relayenergized for a short time interval and increases the time constant ofthe relay control circuit to protect the systern against prematuredrop-out of relay 435 upon momentary interruption of the input signalfrom pickups 17A and 17B.

In other essential respects, the system 36S of FIG. 6 functions inessentially the same manner as the system of FIGS. 14. Thus, if a carentering the retarder is travelling at a speed below the setting oflow-pass filter 24, signal relay 371 is actuated almost immediately andthe retarder is not actuated to its braking condition. Under thesecircumstances, filter 24 is again incorporated in the input circuit tothe noise relay control 422, 423 to provide for de-energization of thenoise relay.

In each of the foregoing embodiments, a major problem is the separationof useful car speed information from extraneous in formation in theinitial signal developed by the transducers such as devices 17A and 17B.This signal inherently includes many harmonics of the fundamentalfrequency; if this extraneous information is not effectively segregatedfrom the desired fundamental signal, the system cannot operate properlyor even safely. The systems of the present invention overcome thisdifiiculty, primarily through the use of the multiple lter circuitsemployed in both the principal signal channel and the noise channel.With the described circuit arrangements, what would appear to -be anunusable mixture of confusing signal information is effectively employedto control car speed and to assure safe and accurate operation of theretarder.

FIG. 7 illustrates a speed-sensitive control system 51S for a railwaycar retarder constituting another embodiment of the present invention.The car retarder controlled by system S18 is slightly different fromthat described and illustrated in FIG. l in that it includes two sets ofretarder rails 13A and 13B that are located imrnediately adjacent eachother longitudinally of the trafc rail 11. The two sets of brakingelements of retarder rails, however, are controlled from a commonretarder actuating mechanism 15. Moreover, the traffic rail 11 is againprovided with a multiplicity of equally-spaced shallow grooves or othersurface discontinuities 16 beginning at a point 11A well ahead of theretarder rails 13A and 13B and ending at the point 11B at the outlet ofthe retarder. Thus, the retarder mechanism is basically similar to thatdescribed above except that the retarder rails lare divided into twolongitudinal segments for convenience in construction. It should benoted that the surface notches or grooves 16 need not be provided in therail 11 along which the retarder rails 13A and 13B are mounted; rather,the other rail of the railway -rnay be provided with the requisitenotches or other surface discontinuities.

Control system 518 comprises three individual pickup devices 517A, 517Band 517C mounted on rail 11. The initial pickup device 517A is locatednear the point 11A at which the individual cars or cuts of cars enterthe retarder system. Pickup device 517B is located within the length oftraffic rail 11 encompassed by the first pair of retarder rails 13A.Pickup 517C is located near the outlet end of the retarder on theportion of the trafiic rail encompassed by the second pair of retarderrails 13B. All three of the pickup units 517A, 517B and 517C areelectrically coupled to an adder amplifier 519. Adder amplifier 519 is asimple adding circuit that combines and amplifies the three initialelectrical signals from the pickup devices 517A, 517B and 517C toproduce at combined initial signal.

Control system 518 does not include an automatic level switch such asthe switch 21 of the previous embodiments. It does comprise a squelchcircuit arrangement comprising an lamplifier 522 having its inputcircuit coupled to the output of adder amplifier 519. Th-e output ofsquelch amplifier 522 is coupled to a rectifier and drive circuit S23that is essentially similar to the rectifier and drive circuit for theautomatic level switch 21 as illustrated in FIG. 2. Thus, drive circuit523 is utilized to energize the operating coil 524 of a squelch relay525.

Relay 525 comprises three sets of contacts, the contacts being shown inthe normal or unenergized position for the relay. The first s-et ofcontacts comprises two fixed contacts 526 and 527 and a movable contact528, movable contact 528 normally being engaged with contact 526. Thesecond set of contacts of `the relay includes a fixed contact 532normally engaged by a movable contact 534, the movable contact engaginga second fixed contact 533 when the relay is energized. The third set ofcontacts for the squelch relay comprises a movable contact 536 that isnormally engaged with a fixed contact 538 but which engages a secondfixed contact 537 upon energization of the relay.

In the first set of contacts of squelch relay 525, the normally closedfixed contact 526 is connected through a resistor 539 to the B+ supplyfor the system. Movable contact 52S is connected to a capacitor 541 thatis returned to ground. The remaining Contact in this set, contact S27,is connected back to the input to recifier and drive circuit 523. Theconnections for the remaining sets of contacts in the squelch relay aredescribed in detail hereinafter.

The output of adder amplifier 519, in addition to its connection tosquelch amplifier 522, is also coupled to an equalizer circuit 542. Theequalizer circuit, which is utilized to compensate for the frequencycharacteristics of the pickup devices 517A, 517B and 517C, as describedmore fully hereinafter, is in turn coupled to a compressor amplifier543. The compressor amplifier may be essentially similar in constructionto the compressor amplifier 22 of the previously described embodiments.Compressor amplifier 543 is also provided with internal `connections tothe contacts 533 and 534 of squelch relay 525 to modify the operatingcharacteristics of the compressor amplifier upon actuation of thesquelch relay, as described more fully hereinafter.

The output of compressor amplifier S43 is coupled to a dual levelclipper gate circuit 545, the two circuits affording an automatic gaincontrol for the system. Gate circuit 545, in turn, is connected througha selector switch 544 to the input to a low pass filter 546. Low

ascenso 25 pass filter 546 performs the same basic function as filtercircuit 24 in the previously described embodiments and may beessentially similar to the construction illustrated in FIG. 3. In thisinstance, however, low pass filter 546 is constructed as a fixed filterand changes in the critical release speed for the retarder controlsystem are effected by switching from one filter circuit to another.Thus, a second low-pass filter 546A, having a different cut-o frequencyfrom filter 546, can be substituted in the operating circuit byactuation of a pair of connecting switches 544 and 544A. Although onlytwo low pass filters are illustrated in FIG. 7, it should be understoodthat several additional filter circuits may be employed with appropriatemeans for switching from one circuit to another for different retarderoperating conditions to establish different desired levels for thecritical release speed of the retarder system. The plural filters andconnecting selector switches afford an adjustably settable filter meansfor setting the release speed for the retarder.

Low pass filter 546 is coupled through selector switch 544A to a driveamplifier 547 that is in turn connected to a rectifier circuit unit 548.The rectifier circuit unit 548 is provided with a main output circuit549 that is utilized to charge a control capacitor 551. The rectifieroutput circuit 549 and capacitor 551 are also coupled to a signal relaycontrol circiut 552. Furthermore, the main output circuit 549 ofrectifier circuit 548 is connected back to the dual lever clipper gate545 to actuate the gate as described more fully hereinafter inconnection with FIG. 9.

The rectifier circuit unit 548 is also provided with a second outputcircuit that is connected to a capacitor discharge circuit 553. Circuit553 affords a controlled discharge of capacitor 551 under certainoperating conditions as described more fully hereinafter.

Signal relay control circuit 552 is utilized to energize the operatingcoil 555 of a signal relay 556. Signal relay 556 includes three sets ofcontacts, all of which are shown in the normal or unenergized conditionfor the relay. The first set of contacts comprises a movable contact 557normally engaged with a fixed contact 553 but engageable with a secondfixed contact 559 when the relay is energized. The second set of signalrelay contacts includes a movable contact 561 normally engaged with afixed contact 562 but movable into engagement with a second fixedcontact 563. The third set of contacts for the signal relay includes amovable contact 565 normally engaged with a fixed contact 566 andengageable with a second fixed contact 567 upon energization of therelay. The movable contact 565 in the third set of contacts for relay556 is connected to the B+ supply for the system. The normally closedfixed contact in this set is connected to capacitor discharge circuit553. The remaining fixed contact 567 is left open-circuited.

In addition to the connection to clipper gate 545, compressor amplifier543 is provided with an output connection to a high-pass filteramplifier 571. This circuit includes, in series, the fixed contact 55Sand the movable contact 557 in the first set of contacts of signal relay556. The remaining contact 559 in this set is left opencircuited.

The output of high pass filter amplifier 571 is coupled to a rectifiercircuit 572 which is in turn coupled to a noise relay control circuit573. The input to control circuit 573 also includes a connection to thefixed contact 538 of squelch relay 525. The related movable contact 536of the squelch relay is returned to system ground, whereas the remainingfixed contact 537 in this set of contacts is left open-circuited.

Control circuit 573 is utilized to actuate a noise relay 575, beingconnected to the operating coil 576 of the relay. Noise relay 575includes two sets of contacts. The first set comprises an open-circuitedfixed contact 577 normally engaged by a movable -contact 57S, thecontact 578 moving into engagement with a second xed contact Y 26 579upon energization of the relay. The second contact set in the noiserelay comprises a fixed contact 531 that is normally engaged by amovable contact 582, the movable contact being engageable with `a secondfixed contact 583 upon actuation of the relay.

In noise relay 575, fixed contact 581 is returned to system groundthrough a resistor 585. Movable contact 582 is connected to a capacitor536 that is returned to ground. Contact 583 is connected back to thenoise relay control circuit 57 3.

The remaining sets of -contacts in noise relay 575 and in signal relay556 are interconnected in a control circuit for the brake relay 27.Thus, the fixed contact 579 of noise relay 575 is directly connected tobrake relay 27. The corresponding fixed contact 577 is leftopen-circuited but the movable contact 57S is connected to fixed contact562 in the signal relay. The related movable contact 561 is connecteddirectly to brake relay 27 whereas the remaining fixed contact 563 isopen-circuited.

With control system 518 in operation, but with no car on traffic rail11, retarder actuating mechanism 15 maintains the retarder rails 13A and13B in their open or released positions. As in the previous embodiments,actuating mechanism 15 is energized from brake relay 27. But theoperating circuit for brake relay 27 is open at contacts 577 and 579 ofnoise relay 575. Accordingly, the retarder remains in its open or offcondition until the brake relay is energized by control system 518.

An approaching car or cut of cars entering the retarder from thedirection of arrow A first engages the notched portion of traffic rail11 at point 11A. Continuing movement of the car causes the traffic railto vibrate at a frequency determined by the velocity of the car or cars.Vibration lof the -rail is detected by pickup devices 517A, 517B and517C, the initial electrical signals from the three pickups beingadditively combined in amplifier circuit 519. At the outset, as thefirst wheels of the cut pass point llA on the rail, the signal frompickup device 517A predominates. t

The output signal from ad-der amplifier 519 is applied to squelchamplifier 522 and to equalizer circuit 542. Referring to squelchamplifier 522, the initial electrical signal from the pickup devices, assupplied by amplifier 519, is amplified further and then applied to'rectifier and drive circuit 523. Circuit 523 in turn energizes relay525, actuating movable contacts 528, 534 and 536 to their alternateoperating positions, closing upon xed contacts 527, 533 and 537respectively.

The closing of contacts 527 and 528 in squelch relay 525 connectscapacitor 541 to rectifier drive circuit 523. Capacitor 541 haspreviously been charged to a substantial potential through theconnection .to resistor 539 and to the B+ supply. Discharge of thecapacitor through the rectifier and drive circuit 523 assures asustained output signal from circuit 523 to hold squelch relay 525energized for .a predetermined time interval, thereby precludingchattering of the relay in :the event of momentary subsequentinterruption of the Isignal immediately following energization of therelay.

The closing of contacts 534 and 533 in squelch relay 525 completes apositive feedback circuit in compressor 'amplifier 543 and materiallyincreases the gain of the comt pressor amplifier. The reason for thiscircuit connection is that it is undesirable to produce a high-amplitudeoutput signal from the compressor amplifier for short-duration signalsand transient-s such .as might be caused by switching along the line, bythe driving of spikes at some nearby point along the same line, or byother sources. With this arrangement, the `gain for compressor amplier543 may be held to la minimal level when there is no car yactually onthe notched portion of traffic rail 11, whereas the gain of theamplifier is increased to a desired operating level once a signal ofsufficient duration is available to -actuate squelch relay 525. In thisregard, 4it should be noted that the rectifier and drive circuit 523 maybe essentially

1. A SPEED-SENSITIVE CONTROL SYSTEM FOR A RAILWAY CAR RETARDER OR THELIKE COMPRISING: A RAIL EXTENDING THROUGH A MAJOR PORTION OF THE LENGTHOF THE RETARDER HAVING A SERIES OF SURFACE DISCONTINUITIES ATPREDETERMINED SPACED INTERVALS ALONG THE RAIL IN POSITION TO BE ENGAGEDBY A RAILWAY VEHICLE WHEEL MOVING ALONG THE RAIL; A PLURALITY OFINDIVIDUAL TRANSDUCERS MOUNTED ON SAID RAIL AT SPACED INTERVALS, FORGENERATING INITIAL ELECTRICAL SIGNALS REPRESENTATIVE OF VIBRATION OF THERAIL, SAID INITIAL SIGNALS INCLUDING REPETITIVE SIGNAL COMPONENTS ATFREQUENCIES DETERMINED CONJOINTLY BY THE SPACING BETWEEN SAID SURFACEDISCONTINUITIES AND THE SPEED OF A VEHICLE MOVING ALONG THE RAIL; ANAMPLITUDE-SENSITIVE SWITCHING DEVICE, CONNECTED TO SAID TRANSDUCERS, FORSELECTING THE INITIAL SIGNAL OF THE GREATEST INSTANTANEOUS AMPLITUDE;FREQUENCY-SELECTIVE CONTROL MEANS, COUPLED TO SAID SWITCHING DEVICE FORSELECTIVELY UTILIZING SAID RE-